1. Field of the Invention
The present invention relates to a voltage regulator circuit, and more particularly to a low drop-out voltage regulator and an adaptive frequency compensation method for the same.
2. Description of the Related Art
Voltage regulators with a low drop-out (LDO) are commonly used in the power management systems of PC motherboards, notebook computers, mobile phones, and many other products. Power management systems use LDO voltage regulators as local power supplies, where a clean output and a fast transient response are required. LDO voltage regulators enable power management systems to efficiently supply additional voltage levels that are smaller than the main supply voltage. For example, the 5V power systems of many PC motherboards use LDO voltage regulators to supply local chipsets with a clean 3.3V signal.
Although LDO voltage regulators do not convert power very efficiently, they are inexpensive, small, and generate very little frequency interference. Furthermore, LDO voltage regulators can provide a local circuit with a clean voltage that is unaffected by current fluctuations from other areas of the power system. LDO voltage regulators are widely used to supply power to local circuits when the power consumption of the local circuit is negligible with respect to the overall load of a power system.
An ideal LDO voltage regulator should provide a precise DC output, while responding quickly to load changes and input transients. Due to the nature of its use in mass-produced products such as computers and mobile phones, LDO voltage regulators should also have a simple design and a low production cost.
A typical LDO voltage regulator consists of a feedback-control loop coupled to a pass element. The feedback-control loop modulates the gate voltage of the pass element to control its impedance. Depending on the gate voltage, the pass element supplies different levels of current to an output section of the power supply. The modulation of the gate voltage is done in a manner such that the LDO voltage regulator outputs a steady DC voltage, regardless of load conditions and input transients.
One problem with traditional LDO circuits is that they are prone to instability. The output section of a traditional LDO circuit includes an output capacitor coupled to the load. This coupling introduces a dominant pole into the feedback circuit. Traditional LDO circuits rely on the equivalent series resistance (ESR) of the output capacitor to restore stability. Within a narrow range of values, the ESR can compensate for the output pole by introducing a zero into the LDO voltage regulator feedback-control loop. Within a range of operating conditions, the zero can increase the phase margin of the LDO voltage regulator.
Unfortunately, the ESR is a parasitic component of the output capacitor and its value cannot easily be determined or controlled to a high precision. The ESR of a capacitor changes significantly with respect to load, temperature, and possibly other factors. If the ESR increases or decreases too much, then the ESR zero will no longer compensate for the pole introduced by the output capacitor.
Another problem with traditional LDO voltage regulators is that the ESR adversely affects the transient response of the LDO voltage regulator. For a LDO voltage regulator to respond rapidly to transients, the ESR must be reduced as much as possible. However, a small ESR will shift the compensating zero of the ESR to a higher frequency, where it will no longer compensate for the pole induced by the output capacitor. In a traditional LDO voltage regulator, the ESR cannot be reduced without threatening the stability of the entire circuit.
Another problem with traditional LDO voltage regulators is that they have a slow transient response under light loads. Under light loads, the frequency of the output capacitor pole decreases. However, the frequency of the stabilizing zero does not change, and the cross-over frequency of the LDO voltage regulator is reduced. Traditional LDO voltage regulators are not designed to enable the stabilizing zero to follow the output pole. If the position of the zero could also be shifted to a lower frequency, the cross-over frequency of the LDO voltage regulator would not be reduced under light loads.
Traditional LDO voltage regulators are prone to instability since the ESR cannot be controlled precisely. Furthermore, their performance suffers degradation under light load conditions. Therefore, there is a need for an improved low drop-out voltage regulator that is suitable for a wider range of capacitive loads while eliminating the minimum ESR restriction of the output capacitor.